Modal filters for modulatable sources

ABSTRACT

A system includes a modulatable source and matched modal filter. The modulatable source is associated with a plurality of source modes to provide a generated signal. The matched modal filter is coupled to the modulatable source to receive the generated signal. Filter modes are to match the source modes. The matched modal filter is coupleable to a link fiber to provide a signal to the link fiber. The filter modes are to match a subset of link fiber modes to couple the plurality of filter modes to the link fiber.

BACKGROUND

A data link may involve a signal source and a legacy fiber. Coupling thesignal source to the legacy fiber may involve a complicated andexpensive alignment procedure, and may result in inefficiencies in thedata link caused by mismatches between the source and fiber.Furthermore, a length of the data link may be limited based on thelegacy fiber and/or the coupling between the source and fiber, resultingin degraded signals having a limited range over the legacy fiber.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

FIG. 1 is a block diagram of a system including a modulatable source anda matched modal filter according to an example.

FIG. 2 is a chart of index profiles and mode powers according to anexample.

FIG. 3 is a block diagram of a system including a modulatable source anda source matched modal filter according to an example.

FIG. 4 is a block diagram of a system including a source coupled to asource matched modal filter according to an example.

FIG. 5 is a block diagram of a fiber assembly including a lens accordingto an example.

FIG. 6 is a block diagram of an air gap connector assembly including alens and aperture according to an example.

FIG. 7 is a flow chart based on a modulatable source and matched modalfilter according to an example.

DETAILED DESCRIPTION

Legacy data link fibers may be coupled with a signal source. However, tooptimally couple the signal source to the lowest order modes of the linkfiber, the link fiber may be associated with an alignment procedure thatis time and resource intensive. For example, a signal source may have adifferent diameter output, e.g., 10 μm, compared to a diameter, e.g., 50μm, 62.5 μm, or other diameter, of the link fiber. The alignment mayinvolve cumbersome use of a camera at the receiving end of the linkfiber to optically verify positioning of the signal source relative tothe end of the fiber, and may involve positioning of a lens between thesignal source and the link fiber. Furthermore, the link fiber may beassociated with generation and propagation of higher-order modes withinthe link fiber that degrade the signal based on a product of bandwidthand length (BW*L product) associated with the link fiber in view of thegenerated signal. For example, a 50 GIF OM4 fiber (OM4 defined inTIA-492-AAAD, “Detail specification for 850-nm laser-optimized, 50-μmcore diameter/125-μm cladding diameter class Ia graded-index multimodeoptical fibers of OM4 performance”) may have a BW*L (bandwidth-length)product of about 4.7 GHz*km. Accordingly, at modulation and/or datarates of 25 gigabits per second (Gbps), the propagation distance islimited to less than 200 meters.

Examples provided herein enable a link fiber to provide a much largerBW*L product, such as 25 Gbps operation over 500 meters, based on amatched modal filter to match a modulatable source. Proper coupling ofthe modulatable source, matched modal filter, and/or link fiber may beverified based on a simple power measurement made at the matched modalfilter and/or the link fiber. The features may be implemented topreserve backward compatibility with legacy link fiber systems. A systemmay include a transmitter and a matched modal filter such as a low-ordermode spatial filter to be coupled to a multimode fiber (MMF) such as a50 μm or 62.5 μm graded-index multimode fiber (50 GIF, 62.5 GIF).

FIG. 1 is a block diagram of a system 100 including a modulatable source110 and a matched modal filter 120 according to an example. Themodulatable source 110 is associated with source modes 112, and thematched modal filter 120 is associated with filter modes 122. Themodulatable source 110 is to provide a generated signal 114 based on thesource modes 112. The generated signal 114 is received at the matchedmodal filter 120. The matched modal filter 120 is to provide a signal126 based on the generated signal 114 and the filter modes 122. Thesignal 126 is to be received at link fiber 130. The link fiber 130 is tobe associated with a subset of link fiber modes 132.

The modulatable source 110 may include various optical sources,including lasers such as edge emitting lasers, single-mode lasers,high-speed Vertical Cavity Surface Emitting Lasers (VCSELs) having highmodulation rates and generating signals at wavelengths such as 780nanometers (nm), 850 nm, 980 nm, 1060 nm, 1300 nm, and other wavelengthsassociated with signal sources.

The matched modal filter 120 enables the source modes 112 of themodulatable source 110 to be matched to the subset of link fiber modes132 of the link fiber 130, enabling efficient coupling of themodulatable source 110 and link fiber 130. For example, the lowest-ordermodes of the link fiber 130 may be launched without launching higherorder modes and without needing cumbersome alignment procedures with themodulatable source 110. The reach of a data link using the matched modalfilter 120 and link fiber 130 may be extended due to the large BW*Lproduct associated with the lowest-order modes of the link fiber 130coupled efficiently to the modulatable source 110 via the matched modalfilter 120. The matched modal filter 120 is to attenuate higher ordermodes, and pass lower order modes, consistent with matching themodulatable source 110, modal filter 120, and link fiber 130 toefficiently couple the modulatable source 110 to the link fiber 130.

The matched modal filter 120 may be a 25 μm, 0.1 radian graded indexfiber (25 GIF), and an index profile of the 25 GIF may be chosen tomatch an index profile of a central core region of the link fiber 130.For example, a 25 GIF patchcord, fiber stub, or pigtail having a curvedrefractive index profile may be used as a matched modal filter forcoupling the modulatable source 110 to the link fiber 130 (e.g., alegacy 50 GIF). The matched modal filter 120 also may be an air-gapconnector, such as an air-gap collimated connector having an aperture toallow lowest-order modes to pass while blocking higher-order modes.

As an additional benefit, the matched modal filter 120 enablesconfirmation of coupling accuracy and the launch/coupling of the lowestorder modes from the modulatable source 110 to the link fiber 130 basedon performing a simple power measurement. For example, a powermeasurement may be taken at the output of matched modal filter 120 toreceive signal 126 (e.g., using a power meter or other device). Thepower measurement may be used to verify the coupling link between themodulatable source 110 and the matched modal filter 120. The powermeasurement may be made at other locations including at the link fiber130, to verify proper coupling between the modulatable source 110,matched modal filter 120, and/or the link fiber 130. However, a complexcamera setup is not needed at the output of the link fiber 130 (e.g., ata receiver) to verify lowest-order mode coupling between the modulatablesource 110 and the link fiber 130. Accordingly, it is possible to verifyproper coupling at the site of the matched modal filter 120 and/ormodulatable source 110, based on a simple power measurement.

FIG. 2 is a chart 200 of index profiles 280, 281 and mode powers 282,284, 286, and 288 according to an example. The mode powers 282, 284,286, and 288 are shown with reference to a distance from fiber center(microns) 290, for a filter spatial refractive index profile 280 and alink fiber spatial refractive index profile 281. The example showncorresponds to a wavelength of approximately 850 nanometers (nm), andthe number of modes can be proportional to the wavelength. In anexample, the filter spatial refractive index profile 280 corresponds toa 25 GIF being used as the matched modal filter, and the link fiberspatial refractive index profile 281 corresponds to a 50 GIF being usedas a link fiber, although other components and/or fibers may be used.Thus, parabolic curvature of the filter spatial refractive index profile280 ends at a distance of +12.5 μm and −12.5 μm from the 25 GIF fibercenter, because the 25 GIF fiber has a diameter of 25 μm. Similarly,parabolic curvature of the link fiber spatial refractive index profile281 ends at a distance of +25 μm and −25 μm from the 50 GIF fibercenter, because the 50 GIF fiber has a diameter of 50 μm. However, inalternate examples, the fiber diameters and corresponding index profilesmay be varied to suit a particular application such as a legacy linkfiber of 62.5 μm diameter or other diameter, with correspondinglydifferent modes and/or refractive index profile curvatures.

A matched modal filter is to couple a transmitter to a link fiber tolaunch low-order modes (e.g., 2-6 lowest order one-dimensional HermiteGaussian modes) from a transmitter into a link fiber. In an example, thematched modal filter may be a 25 μm, 0.1 radian GIF (25 GIF) fiber(e.g., a 25 GIF patchcord or pigtail), whose index profile matches anindex profile of a central core region of a 50 GIF link fiber. Thematched modal filter also may be an air-gap collimated connector whoseaperture passes the lowest-order modes and blocks the higher-ordermodes. The one-dimensional Hermite Gaussian modes also are referred toherein as “modes,” and a number of two-dimensional spatial modes areapproximately equal to the square of the number of one-dimensionalmodes.

As illustrated in FIG. 2, the four modes 282, 284, 286, and 288,associated with the filter spatial refractive index profile 280, aresubstantially matched to the 4 lowest-order modes associated with thelink fiber spatial index profile 281. The filter spatial refractiveindex profile 280 has a parabolic and/or quadratic profile curvaturethat is substantially matched to that of the link fiber spatialrefractive index profile 281. Although four lowest-order modes areshown, 2-6 lowest order modes may be used in alternate examples. Thus,even a standardized simple fiber connection between the matched modalfilter (e.g., 25 GIF and/or air gap connector) and the link fiber (e.g.,50 GIF or 62.5 GIF) can result in exciting and/or launching the 2-6lowest-order modes of the link fiber, due to the matching between thematched modal filter characteristics and the link fiber characteristics(e.g., their refractive index curvatures or other characteristics). Forsystems based on the examples herein, with typical mode coupling along a50 GIF link fiber, the modal dispersion for the four modes illustratedin FIG. 2 will be less than one tenth of the modal dispersion of alegacy 50 GIF not based on the disclosed systems. Thus, an existinglegacy 50 GIF may be enhanced to have a bandwidth*length product similarto a 25 GIF based on the disclosed systems.

The modes 282, 284, 286, and 288 are simplified one-dimensional modelsof the modes contained within a component, such as within the matchedmodal filter and/or link fiber. Thus, references to “mode” throughoutthe specification include references to one-dimensional Hermite Gaussianmodes. Graphs of the one-dimensional Hermite Gaussian modes have acharacteristic symmetric “bell curve” shape including a number ofintensity maxima/minima depending on the order of the mode. First mode282 includes one maximum, second mode 284 includes two maxima separatedby a minimum, third mode 286 includes three maxima separated by twominima, and fourth mode 288 includes four maxima separated by threeminima. Additional modes may include additional maxima/minima. Thehigher-order Hermite Gaussian modes associated with the link fiberspatial index profile 281 (e.g., those below the extension of the filterspatial refractive index profile 280) are not shown in FIG. 2.

FIG. 3 is a block diagram of a system 300 including a modulatable source310 and a source matched modal filter 320 according to an example.System 300 may include a link fiber 330, receiver matched modal filter350, and receiver 360. The system 300 also may include a lens 340 tocouple the modulatable source 310 to the source matched modal filter320. The modulatable source 310 is to provide the generated signal 314based on source modes 312. The source matched modal filter 320 is toreceive the generated signal 314 and provide signal 326 based on thegenerated signal 314, source filter modes 322, and spatial index profile324. The signal 326 is received at the link fiber 330. The link fiber330 is associated with a subset of link fiber modes 332, a link fiberspatial index profile 334, and a connector 336. The link fiber iscoupled to the receiver matched modal filter 350 associated withreceiver filter modes 352. The receiver matched modal filter 350 iscoupled to receiver 360. The example of FIG. 3 shows a receiver matchedmodal filter 350, although in alternate examples the receiver matchedmodal filter 350 may be omitted. The receiver matched modal filter 350may remove higher order modes associated with longer propagation delayscompared to the lower order modes resulting in reduced bandwidth.Omitting the receiver matched modal filter 350 may enable collection ofgreater signal power including the higher order modes, although a lowerbandwidth may potentially result due to the delayed higher order modes.

Modulatable source 310 may be a high-speed Vertical Cavity SurfaceEmitting Laser (VCSEL) to provide the generated signal 314 based onsource modes 312. Accordingly, the source filter modes 322 of the sourcematched modal filter 320 may be chosen to match the source modes 312 ofthe modulatable source 310, thereby allowing for 100% couplingefficiency and maximizing the BW*L product for the signal 326propagating through multimode link fiber 330. Thus, the source matchedmodal filter 320 (and other components, such as connector 336, receivermatched modal filter 350, etc.) may be matched based on modes.

The source matched modal filter 320 (and other components, such asconnector 336, receiver matched modal filter 350, etc.) may be matchedto the link fiber 330 based on refractive index. For example, the sourcematched modal filter 320 may include a refractive index curvature thatis parabolic (e.g., a quadratic index profile). The spatial indexprofile 324 of the source matched modal filter 320 may be matched to thelink fiber spatial index profile 334 of the link fiber 330. In anexample, the spatial index profile 324 of the source matched modalfilter 320 and the link fiber spatial index profile 334 are associatedwith refractive indexes of substantially equal parabolas. Matchingcomponents may enable coupling of the components based on standardconnections. For example, a 25 GIF source matched modal filter 320 maybe butted together with a 50 GIF link fiber 330, without specialadapters therebetween, when the source filter modes 322 match the subsetof link fiber modes 332, and/or the spatial index profile 324 of thesource matched modal filter 320 matches the link fiber spatial indexprofile 334 (e.g., matching refractive index curvatures). Similarly,other components (such as receiver matched modal filter 350, receiver360, etc.) may be coupled efficiently without use of complicatedconnectors.

Connector 336 may be used to couple various components includingsources, filters, fibers, receivers, etc. Connector 336 may be astandard fiber connector to couple together portions of fiber, such assource matched modal filter 320 and/or link fiber 330. Connector 336 maybe a low-order mode spatial filter that may be periodically placed alongthe link fiber 330 or other portions of the system 300, to enable highbandwidth-length (BW*L) modes to travel along the link fiber 330, and toremove any undesirable higher-order modes generated in a fiber to ensurea high BW*L link. Thus, a legacy 50 μm graded-index multimode fiber (50GIF) link may be provided with extended bandwidth-length, and the sourcematched modal filter 320 enables excitation and launching of thecorresponding subset of link fiber modes 332 in the link fiber 330. Theconnector 336 may be a matched modal filter, such as a source matchedmodal filter 320 and/or a receiver matched modal filter 350.

The source matched modal filter 320 and the modulatable source 310 maybe coupled together based on a lens 340 to enable use of differentdiameter modulatable source 310 and source matched modal filter 320. Thelens 340 may adjust a diameter of the generated signal 314 from themodulatable source 310 to match a diameter of the source matched modalfilter 320. For example, the lens 340 may change the diameter of thesource modes 312 to match a diameter of the source filter modes 322, sothat the generated signal 314 includes the same number of modes, and themodulatable source 310 and the source matched modal filter 320 arespatially matched. Although shown separately in FIG. 3, the lens 340 maybe integrated with the source matched modal filter 320 and/or themodulatable source 310.

The link fiber 330 may generate higher-order modes when propagating thesignal 326. The receiver matched modal filter 350 may remove anyundesirable higher-order modes (e.g., generated in the link fiber 330 orelsewhere) from reaching the receiver 360, ensuring a high BW*L link.The link fiber 330 may be a length of over a kilometer and even lessthan a meter. For example, systems including the source matched modalfilter 320 also may include a link fiber approximately a meter inlength, for use with backplane applications to link components of acomputing system such as a server.

The receiver 360 may be backwards compatible with previous generationlinks, e.g., may accept legacy connections such as 50 GIF and/or 62.5GIF. The receiver 360 may be coupled to example systems described hereinbased on the receiver matched modal filter 350. The receiver matchedmodal filter 350 may be a 25 GIF or other low-order spatial filter suchas a connector having an aperture (e.g., air gap connector) to filterout higher-order modes. For example, the receiver matched modal filter350 may filter out all but 2-6 of the lowest order modes. Thus, examplesystems do not impose cumbersome requirements on the receiver 360, andreceiver matched modal filter 350 enables increased flexibility foralignment and coupling.

Systems described herein may include arrays of modulatable sources 310,lenses 340, source matched modal filters 320, link fibers 330,connectors 336, receiver matched modal filters 350, and/or receivers360. For example, a multi-fiber array of link fibers 330 may beassociated with array-based connectors 336, and may be coupled to anarray-based modulatable source 310 including multiple different datalinks carried along multiple channels.

FIG. 4 is a block diagram of a system 400 including a source 410 coupledto a source matched modal filter 420 according to an example. The sourcematched modal filter 420 (e.g., a 25 GIF) is coupled to a link fiber 430(e.g., a 50 GIF, 62.5 GIF, or other legacy fiber such as fiberinfrastructure for data links). The link fiber 430 is coupled to areceiver matched modal filter 450 (e.g., a 25 GIF). The receiver matchedmodal filter 450 is coupled to receiver 460 via connector fiber 472.Components may be coupled based on connectors 436. Source matched modalfilter 420 and receiver matched modal filter 450 are shown as 25 GIF,and link fiber 430 and connector fiber 472 are shown as 50 GIF, althoughother types of components may be used.

System 400 shows fibers (e.g., 25 GIF) used as the source matched modalfilter 420 and the receiver matched modal filter 450. Furthermore,receiver 460 is shown using connector fiber 472 of 50 GIF, althoughother types of connector fiber 472 may be used. In an example, theconnector fiber 472 is a pigtail associated with receiver 460. Inalternate examples, the connector fiber 472 may be integrated within thereceiver 460, and may be omitted such that the receiver 460 is directlycoupled to another fiber and/or connector, including examples where thereceiver 460 is integrated with a connector to receive a link fiberand/or matched modal filter. Use of fibers enables simple connectors 436to be used, for coupling the source 410 to the link fiber 430 and thereceiver 460. Thus, system 400 illustrates an example of efficientlycoupling a source 410 to a legacy link fiber 430, using a backwardcompatible receiver 460. The receiver 460 is backward compatible becauseit is coupleable directly to a legacy fiber such as 50 GIF (or 62.5 GIF)based on the connector fiber 472 (shown as 50 GIF, but connector fiber472 may be other types of fiber to match other legacy fibers such as62.5 GIF). The receiver matched modal filter 450 enables efficientcoupling to the connector fiber 472 and the receiver 460.

FIG. 5 is a block diagram of a fiber assembly 500 including lens 540according to an example. A modulatable source 510 (e.g., a VCSEL) iscoupled to the assembly 500 via a lens 540 and a matched modal filter520 (e.g., a 25 GIF), and the assembly 500 is coupled to a link fiber530 (e.g., a 50 GIF). For example, the matched modal filter 520 iscoupled to link fiber 530 via connector 536. The fibers may be connectedbased on a butt connection where matched modal filter 520 is buttedagainst link fiber 530. The assembly 500 may use lens 540 to couple themodulatable source 510 and link fiber 530, although other techniques maybe used to couple the modulatable source 510 and other components. Thefiber assembly 500 is associated with a diameter d 576 and numericalaperture NA 578.

The matched modal filter 520 is shown as a 25 GIF, and the link fiber530 is shown as a 50 GIF, although other types of fibers may be used.For example, fibers based on compatible combinations of d and NA ford*NA products may be used, regarding d*NA of the modulatable source 510,matched modal filter 520, and at least a portion of link fiber 530. Forexample, the modulatable source 510 may be associated with approximately4 modes, based on a product of diameter and numerical aperture d*NA of10 μm*0.25 radian, at a wavelength of approximately 850 nm (e.g.,corresponding to a wavelength of light emitted from VCSEL source 510).The lens 540 enables an air gap in the fiber assembly 500, and couplingto the matched modal filter 520. The matched modal filter 520 is tofilter out any higher modes of the modulatable source 510, ifapplicable, and efficiently couple those modes to the link fiber 530.Specifically, the matched modal filter 520, shown in FIG. 5 as the 25GIF fiber, efficiently couples approximately four of the lowest ordermodes to the link fiber 530, based on a product of diameter andnumerical aperture d*NA of 25 μm*0.1 radian for the 25 GIF. The diameterd 576 and the numerical aperture NA 578 may be chosen to provideapproximately 2-6 of the lowest order modes to the link fiber 530. Thus,the matched modal filter 520 may be a fiber other than the 25 GIF asshown, including a 30 GIF or 20 GIF etc. The actual diameter d 576 mayinclude a range of values, along with a range of values for numericalaperture NA 578 (and, thus, the d*NA product). In an example, d 576 andNA 578 may be increased slightly in view of handling bending losses.

A product of bandwidth and length (BW*L product) associated with a datalink may be extended based on the components described herein. A matchedmodal filter, such as the fiber assembly 500 of FIG. 5, may extend theBW*L product based on a d*NA product associated with a reduced numericalaperture. For example, the BW*L product is proportional to a bit rateand length (BR*L product). For the examples described herein, the bitrate BR*L product is related to the numerical aperture NA of the gradedindex link fiber 530, as follows:

${{BR} \cdot L} \propto \frac{1}{{NA}^{4}}$

As shown in the equation above, at a given bit rate BR the propagationlength L is inversely proportional to the 4^(th) power of the numericalaperture NA. Thus, a 2× decrease of the NA, e.g., from 0.2 rad to 0.1rad, can potentially increase the BR*L product (and thus an associatedBW*L product) by 16×. In an example, for NA=0.2 rad, BR·L˜2.3 Gbps·km ata wavelength of approximately 850 nm. For NA=0.1 rad, BR·L˜35 Gbps·km ata wavelength of approximately 850 nm. Although filtering in FIG. 5 isbased, at least in part, on a fiber, other techniques may be used forfiltering, including a lens/aperture assembly or diffractive elementsuch as a holographic diffraction pattern and the like.

FIG. 6 is a block diagram of an air gap connector assembly 600 includinglens 640 and aperture 674 according to an example. A connector fiber 672is coupled to the assembly 600, and the assembly 600 is coupled to alink fiber 630. The assembly may use a collimated signal based on lenses640 and apertures 674. The air gap connector assembly 600 is associatedwith a diameter d 676 and numerical aperture NA 678.

The connector fiber 672 is shown as a 50 GIF, although other types offibers may be used. The connector fiber 672 is associated withapproximately 18 modes, based on a product of diameter and numericalaperture d*NA of 50 μm*0.2 radian. The lenses 640 and apertures 674enable the air gap connector assembly 600 to filter out approximately 14higher modes of the approximately 18 modes associated with the connectorfiber 672. Specifically, the air gap connector assembly 600 shown inFIG. 6 passes approximately four of the lowest order modes to the linkfiber 630, based on a product of diameter and numerical aperture d*NA of25 μm*0.1 radian. The diameter d 676 and the numerical aperture NA 678may be chosen to provide approximately 2-6 of the lowest order modes tothe link fiber 630. The actual diameter d 676 may include a range ofvalues, along with a range of values for numerical aperture NA 678 (and,thus, the d*NA product). In an example, d 676 and NA 678 may beincreased slightly in view of handling bending losses. Although twomatched lenses 640 and two matched apertures 674 are shown, the lenses640 and/or the apertures 674 may differ or otherwise be unmatched fromeach other. For example, first and second lenses may be used with firstand second apertures.

FIG. 7 is a flow chart 700 based on a modulatable source and matchedmodal filter according to an example. In block 710, a signal isgenerated from a modulatable source associated with a plurality ofsource modes. For example, the modulatable source may be a VerticalCavity Surface Emitting Laser (VCSEL). In block 720, the plurality ofsource modes are coupled to a matched modal filter based on matching aplurality of filter modes to the plurality of source modes to receivethe generated signal. For example, the matched modal filter may be a 25GIF fiber, an air gap connector, or other filter. In block 730, thematched modal filter is coupled to a link fiber to provide the signalfrom the matched modal filter to the link fiber, wherein the filtermodes are to match a subset of link fiber modes of the link fiber. Forexample, the filter modes are to match based on the lowest 2-6one-dimensional Hermite Gaussian modes of the link fiber. The filtermodes also may match based on parabolic curvature of refractive indexprofiles, such as a match between a 25 GIF fiber filter and a 50 GIFlink fiber. In block 740, 2-6 lowest-order modes of the link fiber arelaunched to transmit the signal. For example, the modes are launchedbased on excitation at the link fiber caused by a signal emitted by amode filter. In block 750, proper coupling between the modulatablesource and matched modal filter is identified based on a powermeasurement. The power measurement may be taken at the matched modalfilter, for example.

What is claimed is:
 1. A system comprising: a modulatable sourceassociated with a plurality of source modes to provide a generatedsignal; and a matched modal filter coupled to the modulatable source toreceive the generated signal, wherein a plurality of filter modesassociated with the matched modal filter are to match the plurality ofsource modes to couple the plurality of source modes to the matchedmodal filter; wherein the matched modal filter is coupleable to a linkfiber to provide a signal to the link fiber, wherein the filter modesare to match a subset of link fiber modes associated with the link fiberto couple the plurality of filter modes to the link fiber, and whereinthe filter modes comprise at least 4 lowest-order one-dimensionalHermite Gaussian modes corresponding to at least 4 lowest-orderone-dimensional Hermite Gaussian modes of the link fiber.
 2. The systemof claim 1, wherein the matched modal filter comprises a spatial indexto match a spatial index associated with the link fiber.
 3. The systemof claim 1, wherein the matched modal filter comprises a multi-modeGraded-Index Fiber (GIF) including a substantially parabolic spatialrefractive index profile.
 4. The system of claim 1, wherein the matchedmodal filter comprises an air-gap collimated connector assemblyincluding a connector fiber, lens, and aperture.
 5. The system of claim1, wherein the matched modal filter comprises a diameter d ofapproximately 25 microns, a numerical aperture NA of approximately 0.1radians, and a parabolic refractive index.
 6. The system of claim 1,wherein the matched modal filter is to spatially match the modes of themodulatable source with the use of a lens to adapt a diameter of themodulatable source with a diameter of the matched modal filter.
 7. Thesystem of claim 1, further comprising a receiver matched modal filtercoupleable to the link fiber, including receiver filter modes to matchthe source modes and the subset of link fiber modes.
 8. The system ofclaim 1, wherein the modulatable source includes a Vertical CavitySurface Emitting Laser (VCSEL).
 9. A method, comprising: generating asignal from a modulatable source associated with a plurality of sourcemodes; and coupling the plurality of source modes to a matched modalfilter based on matching a plurality of filter modes to the plurality ofsource modes to receive the generated signal; wherein the matched modalfilter is coupleable to a link fiber to provide a signal from thematched modal filter to the link fiber, and the filter modes are tomatch a subset of link fiber modes of the link fiber; wherein the filtermodes comprise at least 4 lowest-order one-dimensional Hermite Gaussianmodes corresponding to at least 4 lowest-order one-dimensional HermiteGaussian modes of the link fiber.
 10. The method of claim 9, furthercomprising launching the at least 4 lowest-order modes of the link fiberto transmit the signal.
 11. The method of claim 9, further comprisingidentifying proper coupling between the modulatable source and matchedmodal filter based on a power measurement.
 12. A system, comprising: amodulatable Vertical Cavity Surface Emitting Laser (VCSEL) source toprovide a generated signal based on a plurality of source modes; asource matched modal filter associated with a plurality of source filtermodes to match the plurality of source modes, the source matched modalfilter coupled to the modulatable source to receive the generated signaland provide a signal from the source matched modal filter; a link fiber,coupled to the source matched modal filter to receive the signal,associated with a subset of link fiber modes to match the plurality ofsource filter modes and the plurality of source modes, wherein thesource matched modal filter includes a spatial index profile to match alink fiber spatial index profile, and wherein the filter modes compriseat least 4 lowest-order one-dimensional Hermite Gaussian modescorresponding to at least 4 lowest-order one-dimensional HermiteGaussian modes of the link fiber; and a receiver matched modal filtercoupled to the link fiber, including a plurality of receiver filtermodes to match the plurality of source modes and the subset of linkfiber modes.
 13. The system of claim 12, further comprising an air gapconnector to connect the link fiber to propagate the signal.
 14. Thesystem of claim 12, wherein the link fiber is utilized in a backplane ofa computing system using an air gap connector.